We have an existing DAG combine for when an insert/extract subvector pair is entirely a nop, but we hadn't handled the case where the net result was either an insert or an extract (but not both). The transform is restricted to index = 0 to avoid having to adjust indices after the transform.
Reviews, a couple comments on the test changes:
- Mostly RISCV, mostly schedule reordering.
- One real regression in splats-with-mixed-vl.ll due to a different overly aggressive combine, fix in a follow up patch.
- The test/CodeGen/X86/vector-replicaton-i1-mask.ll diff looked concerning at first, but not the mask size at most 4 i1s. I think the type changes on the mask loads are correct, but would welcome a second opinion with someone more familiar with AVX512 codegen.
Extra period at the end.